This invention generally relates to an apparatus for the application of a conductive adhesive medium in a fluid state, such as solder, to the side printed with circuitry of a board bearing electronic components. The apparatus includes a hollow nozzle standing in a supply tank for the adhesive medium, and an obliquely rising conveyor belt for one circuit board at a time, wherein the upper end of the hollow nozzle possesses two overflow edges arranged perpendicular to the direction of transport for the formation of a wave, through the crest of which the side of the card, previously moistened with a flux, is passed.
A soldering apparatus is known which is provided with two nozzles, which are arranged one behind the other with respect to the direction of transport of the circuit boards to be processed. The purpose of the first nozzle is to apply the adhesive medium or solder, generally a tin alloy, to the underside of the circuit board in a sufficient quantity to ensure covering of all the areas to be soldered, while the function of the second nozzle is to remove the surplus solder.
It is already known that the apparatus can be embodied with a single soldering nozzle, with which both functions can be achieved. However, with decreasing dimensions of the areas to be soldered, it is becoming increasingly difficult to achieve 100% adhesion to all the areas using the conventional single-nozzle apparatus.
The present invention has for its aim to procure an apparatus with which a 100% coverage of the areas to be soldered is in fact achieved.
The apparatus according to the invention is distinguished in that near to the leading overflow edge with respect to the direction of transportation, an adjustable damming means is arranged for the temporary raising of the fluid level.
Owing to the damming means at the front of the nozzle, an asymmetrical, when viewed in vertical section, wave crest is achieved, which brings in train the following unexpected advantages.
1. The underside of the circuit board is guided through a heightened crest over the commencement of the nozzle, which effectuates an intensified flow of adhesive medium.
2. The quality descending rear flank of the wave-crest results in a gradual decrease in surplus material, whereby short-circuit bridges are prevented.
According to a further development of the apparatus, the invention proposes embodying the damming means in the form of a channel arranged perpendicular to the direction of transportation, the fluid adhesive medium being pumped upwards into said channel, the whole being arranged so that, together with the wave emerging from the nozzle, the asymmetrical wave pattern is achieved.
In a particular embodiment, as the pumping means, a rotatably journalled driven roller with a non-round or round but eccentric transverse cross section is arranged in the longitudinal direction of the channel.
This embodiment offers the unexpected advantage that a small extra wave, whose frequency depends on the speed of rotation of the roller, is superimposed on the crest of the asymmetrical wave. Through this an agitated waveform is brought about, which is particularly advantageous for the soldering of very small components on the underside of the circuit board, since with a smooth wave-crest surface such components are difficult to reach, on account of the high surface tension of the adhesive medium. Owing to the fact that this takes place in the leading, downflow wave part, the portion of the wave which is situated above the nozzle is hardly disturbed, whereby the smooth surface which is necessary for the gradual decrease in the surplus adhesive medium is maintained.
The foregoing and other objects, features and advantages of this invention will be apparent from the following, more particular, description of the preferred embodiments of this invention, as illustrated in the accompanying drawings.